Amazon hasn’t released sales for any of its Kindle devices beyond saying in January that units sold in the nine-week period ending Dec. 31 increased 177 percent from the same period the previous year. The Seattle-based company said the Kindle Fire was the best-selling item on its site at the time.

That has made it difficult for analysts to pin down a number. Ross Sandler, at RBC Capital Markets, has predicted 3 million to 4 million units were sold in the fourth quarter, while Anthony DiClemente at Barclays Capital says it’s 5 million and Jordan Rohan at Stifel Nicolaus & Co. projects as many as 6 million.

“It’s really difficult to get an actual number,” said Brian Nowak, an analyst at Nomura Securities in New York, who estimates Amazon sold about 5 million Kindle Fires in the fourth quarter. “You talk to the different suppliers and you read through what IDC and all those guys say, but it is hard.”

Nowak said the numbers shouldn’t deter investors as long as they give a sense of whether sales are likely to go up or down and indicate how Amazon is faring against its competitors — Apple Inc. and Samsung Electronics Co.